Studies of the BB isoenzyme of creatine kinase (CK-BB) using RIA in the serum of patients with lung and prostate cancer were completed. CK-BB did not appear to be useful marker for response to therapy's in patients with advanced prostate cancer. In small cell lung cancer, serum CK-BB concentrations did seem to reflect response to therapy particulary in those patients with extensive disease who improved. Rising CK-BB levels were observed in relapsing patients. New studies were initiated of CK-BB levels in clones of small cell lung cancer cells with the intention of examining the heterogeneity of CK-BB levels and substrate levels in various cells within a tumor.